Rheometer for rapidly measuring small quantity samples

ABSTRACT

The present invention relates to a parallel rheometer and improved force sensor elements, which may advantageously be used in combination with the parallel rheometer. The parallel rheometer of the present invention allows simultaneous measurements of a plurality of samples so as to allow characterization of a plurality of samples within a short time period. The force sensor element according to the present invention allows simultaneous measurement of a shear force and a normal force applied to the sensor element. Moreover, the force sensor may employ stress-optic material.

TECHNICAL FIELD

The present invention relates to apparatus and methods for measuringphysical properties of samples when subjected to an external shearforce.

BACKGROUND

Producing materials having specific required properties is steadilygaining in importance and, hence, the field of combinatorial chemistry,which generally refers to methods for creating collections of diversematerials or compounds, commonly known as libraries, is steadilyincreasing and has revolutionized the process of drug discovery.Combinatorial chemistry enables researchers to rapidly discover andoptimize useful materials such as polymers, superconductors, magneticmaterials, etc. In order to record the various properties of thematerials obtained by combinatorial chemistry, it is necessary toprecisely and efficiently determine the characteristics of thematerials, preferably under varying environmental conditions. Oneimportant and useful characteristic is the behavior of a material, inparticular of a polymer, when exposed to an externally-applied shearforce. Instruments for measuring the response of fluids to applied shearare generally referred to as rheological instruments, which may besubdivided into indexers and rheometers. Indexers measure a quantitywhich is correlated with the rheological characteristics, but which isdifficult to analyze in terms of intrinsic material properties. Althoughindexers can be assembled rapidly from commercially availablecomponents, the results of measurements carried out by means of suchindexers are difficult to relate to results of measurements from otherindexers, or to intrinsic material properties, without extensivecalibration.

Rheometers, on the other hand, measure intrinsic materialcharacteristics, giving them broad applicability. Such generality,however, comes at a price due to design costs and complexity which aredictated primarily by the need for well-defined static and dynamic testconditions. Moreover, these known rheometers require fairly largequantities of sample in the order of 500 mg so as to obtain the requiredaccuracy in analyzing the samples. Such large quantity samples, however,are usually not provided by combinatorial synthesizing methods in which,generally, a large amount of differing samples of small quantity areproduced.

Rheological measurements on sample materials, such as polymericmaterials, are performed in their simplest geometry such that the sampleis placed between two parallel plates of a design area separated by agap of known distance, wherein the sample is sheared by applying a forceto one of the plates while keeping the other plate fixed. This resultsin a displacement, i.e. a deformation of the sample confined between theplates, which can be characterized in terms of the shear stress and theshear strain. From these quantities and the dimensions of the sample, ashear modulus may be calculated. In general, the shear modulus is afunction of the sample history, the shear strain and the strain rate.For polymeric materials, the temporal dependence of the shear modulus atconstant stress typically exhibits four different regimes reflectingdifferent relaxation mechanisms available to the polymer chain.

For sufficiently small deformations, most polymers exhibit linearviscoelastic behavior in which the shear modulus is independent of theshear strain. Theories of polymer dynamics generally explain theresponse of a chain in terms of normal modes, each having acharacteristic frequency. The linear viscoelastic theory gives, then,the response of the material as a function of shear history. Inmeasuring the mechanical property of a sample material, the sample issubjected to a varying force, e.g. a sinusoidal-varying force, and theresulting deformation, i.e. the response of the sample is observed. Thefrequency response of the sample may then be analyzed in accordance withviscoelastic theories to obtain information on the requiredcharacteristic of the material.

In order to perform these measurements with a high degree of accuracy,the rheological apparatus must be capable of producing a well-defineddisplacement within a specified frequency range. Additionally, sinceeach frequency corresponds to probing the response of the samplematerial at a particular relaxation time, such measurements take arelatively long time period when the relaxation mechanism of the samplesrequires the employment of low frequencies. Hence, measuring a pluralityof samples which may be produced by combinatorial chemistry is a verytime-consuming and therefore very expensive procedure.

Moreover, the performance of accurate measurements requires theapplication of suitable sensor elements for detecting the shear stressin the samples. In order to obtain meaningful experimental results, thesensor elements have to be suitably designed so as to reflect theresponse of the sample to the applied shear strain without anyinterference or at least minor interference of the sensor element.

In view of the above-mentioned problems, it is an object of the presentinvention to provide apparatus and methods for rheological measurementswhich are capable of producing reliable measurement results for aplurality of small quantity samples within a short time period.

SUMMARY OF THE INVENTION

According to one aspect of the present invention, there is provided aminiature rheometer for analyzing a small quantity sample of a materialof interest, wherein the rheometer comprises a first plate and a secondplate, forming a pair of plates having a known geometry for confiningthe sample between the plates, said sample having a volume of 200microliters or less; an adjusting device for adjusting the separation ofthe plates; an actuating element mechanically coupled to the firstplate, which produces a shear strain within the sample by generating adefined small-scale relative motion of the first and second plates; asensing element which outputs a position signal indicative for adisplacement of at least one of the first and second plates; and afeedback circuit for providing force rebalance of the force, applied tothe sample by the small-scale relative motion of the first and secondplates, on the basis of the position signal, wherein an amount of forcerebalance is a measure for the stress within the sample.

The miniature rheometer according to the present invention is suitablyadapted to characterize combinatorial materials. In contrast to knowninstruments which generally require large sample volumes, typically10-100 times the quantity produced by current combinatorial syntheticapproaches, the present invention merely requires sample quantitieshaving a volume of 200 microliters or less, or preferably 10-50microliters, which are easily obtained by combinatorial synthesizingmethods. Since measurements on such small sample volumes requires anaccurate response of the rheometer on a corresponding small lengthscale, the present invention provides a sensing element and a feedbackcircuit which provide for force rebalance of the force that is appliedto the sample in order to avoid the undesired inherent displacement ofconventional force sensor elements due to shear forces exerted by thesample on the force sensor. Accordingly, the force balance may becontrolled such that the present miniature rheometer exhibits anextremely high effective stiffness with respect to the sample, which inturn insures accurate measurement results, even at very smalldisplacements.

In further embodiments, the position signal which is input into thefeedback circuitry so as to adjust the force rebalance is obtained fromthe sensing element which may comprise a deformation-sensing element, aencoder means, or any other appropriate means suitable to determine thelocation of the plates with a spatial resolution that is substantiallysmaller than a minimal displacement of the plates as required for thedesired measurement accuracy. The rheometer may comprise asensor-actuating element that is mechanically coupled to the secondplate so as to maintain the second plate at a predefined position uponreception of a driving signal from the feedback circuit. Alternatively,the actuating element may be driven from the feedback circuit such thatthe first plate maintains a desired displacement, wherein the secondplate may be a fixed plate or may be kept at a fixed position.

In a further embodiment, the actuating element used as a shear strainproducing means comprises a piezo-electric actuator so as to produce thesmall-scale relative motion. This allows the miniature rheometer tocreate small relative displacements of the plates while insuring easycontrol and configuration of the actuator.

In a further embodiment, the plates are disposable plates. Theemployment of disposable plates may considerably facilitate samplepreparation and sample replacement after completion of a measurementrun.

Advantageously, the deformation sensing element is coupled to theactuating element or the sensor-actuating element or to both and detectsa deformation of at least a portion of the respective actuating element.The deformation sensing element measures the amount of deformationgenerated by the shear force applied to the sample. In this manner, thesignal from the deformation sensing element may either be directly usedas a measure for the shear force, or it may be supplied to the feedbackcircuit which adjusts the force exerted on the deformation sensingelement by the actuating element or the sensor-actuating element so asto return the deformation sensing element to a predefined location, e.g.the undeflected state of the deformation sensing element.

In a further embodiment, two or more miniature rheometers may bearranged so as to form a parallel rheometer for simultaneously measuringtwo or more small quantity samples. To this purpose, a common controlunit is provided which controls the shear strain producing means and theforce sensors of the two or more miniature rheometers.

According to a second aspect of the present invention, there is provideda parallel rheometer for simultaneously analyzing materialcharacteristics of two. or more samples, wherein the parallel rheometercomprises first and second plates respectively having regions forreceiving and confining said two or more samples, the first and secondplates being moveable relative to each other; an actuator adapted tomove the first and second plates relatively to each other for producinga shear strain within each sample; and at least one sensor associatedwith each region for simultaneously detecting shear stress within eachsample.

As previously stated, standard rheological measurements oftencharacterize materials according to their frequency response to anapplied oscillatory shear force. Here, the frequencies of interest setthe minimum measurement time required, which is typically three or fourtimes the reciprocal of the frequency. Thus, by allowing thesimultaneous measurement of a large number of samples by means of aparallel rheometer according to the present invention, a quick screeningof a plurality of material samples (such as those produced bycombinatorial synthetic approaches) is feasible. Moreover, according tothe present invention, the minimum sample volume may be kept smallerthan in known single-channel rheometers so as to permit measurements asa function of environmental conditions to use much faster conditionchange rates than are possible with large samples. To this end, theparallel rheometer as well as the miniature rheometer may comprise meansfor applying varying environmental conditions. Preferably, theenvironmental conditions to be varied, individually or simultaneously inany combination, at least include temperature, pressure at a fixed gascomposition, composition of a gas atmosphere surrounding the sample,electric field, magnetic field, and time of application of one or moreof the preceding quantities when adjusted to respective predeterminedvalues. The means may be designed so as to allow the variation of theenvironmental conditions individually for each sample and/orsimultaneously for a group of samples.

In a further embodiment, the shear stress detector comprises amicromachined sensor element at each sample position. This allows massproduction of nearly identical sensor elements at low cost, wherein therequired sample volume may easily be maintained relatively small due tothe reduced sensor mass and increased sensitivity provided bymicromachined devices.

According to a third aspect of the present invention, there is provideda rheometer, comprising: a pair of plates spaced apart from each otherby a defined distance for receiving and confining a sample therebetween,an adjusting means which adjusts the distance between the plates, adriving means coupled to at least one of the plates, which generates arelative motion between the plates without changing the distance, and ashear stress sensor, the shear stress sensor comprising a stress-sensingmaterial of a defined stress-optic coefficient indicating one ofbirefringence and retardation of linearly polarized light passing thestress-sensing material, as a function of applied stress/unit pathlength.

The shear stress detector comprises a stress-optical sensor elementwhich is insensitive to electric and magnetic fields, and thus allows toanalyze samples within such fields without creating electrical noise.

According to a fourth aspect of the present invention, there is provideda sensor element for outputting a signal in response to a mechanicaldeformation applied to the sensor element, wherein the sensor elementcomprises: a sample plate arranged within an opening of a substrate; atleast two tethers, one of each tether being attached to the sampleplate, the other end of each tether being attached to the substrate soas to support the sample plate; a piezo-resistive portion in each of thetethers; and a wiring line formed on the tethers and the substrate,connecting each piezo-resistive portion with a corresponding contact padformed on the substrate, wherein the piezo-resistive portion of one ofsaid at least two tethers is adapted to generate a maximum change of itsinternal resistance when a shear force is applied to the sample plate,and wherein the piezo-resistive portion of the other one of said atleast two tethers is adapted to generate a maximum change of itsinternal resistance when a force normal to the sample plate is applied.

As is generally known, shearing a viscoelastic material also generates aforce along the shear gradient direction. Such forces are potentially ofeither sign and can be comparable in magnitude to the shear force. Theycan strongly affect the macroscopic flow properties of a material. At aminimum, rheometers must be sufficiently stiff in the shear gradientdirection so that shear actuation results in as near a pure shear fieldas possible, and that any strain determination measures only the shearstrain. By means of the sensor element provided according to the thirdaspect of the present invention, a shear force and a normal forceapplied to a sample can be detected simultaneously. This improves theaccuracy of the measurement results due to separation of the totalinstrumental response into shear and normal force components andprovides a more comprehensive picture of the response of the sample toapplied shear.

This sensor element is particularly advantageous when used incombination with the above parallel rheometer.

Further advantages and objects of the present invention follow from thedependent claims and the detailed description of the preferredembodiments

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic perspective view of a translational embodiment ofthe miniature rheometer according to the present invention.

FIGS. 2a-2 c are schematic top views of actuator elements shown in FIG.1.

FIG. 3 is a schematic perspective view of a rotational embodiment of theminiature rheometer according to the present invention.

FIG. 4 is a schematic top view of an actuator element as shown in FIG.3.

FIG. 5 is a schematic top view of a force sensor which may be used inthe rotational embodiment as shown in FIG. 3.

FIG. 6 is a schematic top view of a further force sensor, which may beused with the rotational embodiment as shown in FIG. 3.

FIG. 7 is a schematic side view of an individual measurement element ofa translational embodiment of the parallel rheometer using micromachinedsensor elements.

FIG. 8 is a schematic top view of the individual measurement element ofthe translational embodiment of the parallel rheometer as shown in FIG.7.

FIG. 9 is a schematic perspective of a rotational embodiment of theparallel rheometer according to the present invention.

FIG. 10 is a schematic perspective view of an optical shear forcesensing element which may be used in the parallel rheometer according tothe present invention.

FIG. 11 respectively shows a further arrangement for optically detectingthe shear force within a sample in a parallel rheometer according to thepresent invention.

FIG. 11A shows an alternative arrangement for optically detecting theshear force within a sample in a parallel rheometer according to thepresent invention.

FIG. 12 schematically shows the structure of a sensor element whichallows the simultaneous measuring of a shear force and a normal forceapplied to a sample, wherein the sensor element is preferably useable ina parallel rheometer according to the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

With reference to FIGS. 1 and 2, a translational embodiment, i.e. anembodiment that produces a translational displacement between a topsurface and a bottom surface of a sample, of the miniature rheometeraccording to the present invention will now be described in detail.

In FIG. 1, merely a portion of the translational embodiment of theminiature rheometer of the present invention is shown. A moving plate102 is mounted on a shaft 101. Another plate 100, which will also bereferred to as the “fixed plate” even though this plate may be movable,is arranged parallel to the moving plate 102. Plates 100 and 102 havesurfaces which are arranged in an opposed relationship so as to beparallel to each other. The geometrical structure as well as thedimensions of the respective surfaces are selected so as to result in adesired configuration. Although the plates 100 and 102 are shown ascircular elements, any other geometrical structure, such as squares orrectangles, etc., may be used for the surfaces of plates 100 and 102.Plate 100 is also mounted on a shaft 103. Preferably, the shafts 101 and103 are made of a rigid, thermally insulating material. However, anyappropriate material may be used, and in some cases electrical and/orthermal conductivity of portions of the shaft 101 and 103 may bedesirable. Shaft 101 is attached to an actuating element 104, whichcomprises a metal foil 105 which is of a rectangular shape. On at leastone side of the metal foil 105, a plate of piezoelectric material 106 isattached. In this embodiment two piezoelectric plates 106 are used withthe metal foil 105, however, it is also possible to use merely onepiezoelectric plate on one surface of the metal foil 105. Moreover, twoor more piezoelectric plates may be used in accordance with designrequirements. Actuating element 104 is attached to shaft 101, preferablyin a non-permanent manner, e.g. by providing a slit for receiving theactuating element 104. This permits the use of disposable plates, whichin turn facilitates measurements of samples that are difficult to removefrom the plates. The actuating element 104 is further attached to amechanical assembly, which may be provided in the form of clampingblocks 107 which comprise slits for receiving end portions of theactuating element 104. A wiring assembly for applying a voltage to thepiezoelectric plates 106 is also provided, but is not shown in FIG. 1.Depending on the crystallographic orientation of the piezoelectricplates 106, metal foil 105 may serve as a common electrode for applyinga desired voltage to the plates 106. Alternatively, metal foil 105 maybe covered by a thin, electrically-insulating layer on the middleportion, where the piezoelectric plates 106 cover the metal foil so thata voltage may be applied to the piezoelectric plates 106 by means of theelectrically conductive end portions of the metal foil 105, which areclamped in place by a clamping blocks 107, which then preferably may bemade of an electrically conductive material so as to serve electrodes.

In this embodiment, the assembly regarding the fixed plate 100 isidentical to the structure referring to moving plate 102, i.e. shaft 103is attached to an actuating element 108 comprising a metal foil 109 withpiezoelectric plates 110 on either side. Clamping blocks 111 areprovided in order to clamp the actuating element 108 in place. Regardingelectrical connections to the piezoelectric plates 110, the wiring isconfigured in an analogous manner as previously described with referenceto plates 106.

FIG. 2a is a schematic top view of the actuating element 104 or of theactuating element 108 of FIG. 1. For the sake of clarity only, thereference signs regarding actuating element 104 are shown. As thepiezoelectric plates 106 are made of piezoelectric material, the plates106 will undergo dimensional changes in response to an applied electricfield. In FIG. 2a, the piezoelectric plates 106 are arranged such thatthe application of a voltage of known polarity will cause a contractionof the plate along the longitudinal axis as indicated by correspondingarrows in the figure.

FIG. 2b shows the actuating element 104 in the state when a voltage isapplied.

FIG. 2c schematically shows the bent actuating element 104, when the endportions thereof are clamped in place by the clamping blocks 107. Theapplication of a voltage to the clamped actuating element 104 produces aquadruple bend that is symmetric about the center portion of theactuating element. Reversing this voltage reverses the bend. Applicationof an alternating voltage to the actuating element 104 therefore resultsin reciprocating linear translation of the center of the actuatingelements as is indicated by arrow 112.

In operation, a sample of a material of interest, preferably a materialproduced by combinatorial synthetic approaches, is disposed in the gapbetween the moving plate 102 and the fixed plate 100. In order to obtainwell-defined experimental conditions, means may be provided to removeexcess sample material, which may have accumulated during sampleloading, from the edges of the plates. This is typically accomplished byproviding a sharpened punch which translates and/or rotates along theplate edges so as to cut away excess material which extends beyond theedges of the plates. After having arranged the samples, which can beprepared by molding or otherwise forming samples to the dimensions ofthe plates 100 and 102, the plates 100 and 102 are translated so as tobe brought into contact with the formed sample. For this purpose, meansare used which is not shown in the figures, but, which may, as theperson skilled in the art will readily appreciate, be any appropriatemechanical assembly used in this field for translating the plates alonga line normal to the opposing surfaces of the plates, so as to correctlyadjust and define the distance of the gap. Alternatively, material maybe placed on one plate and the other plate may be translated to adefined relative gap distance in order to mold specimens of knownthickness. In this case, excess sample material is removed as previouslypointed out.

Next, a defined voltage is applied to the actuating element 104,preferably an alternating voltage with known rate and amplitude, so asto achieve a desired displacement of the middle portion of the actuatingelement 104, as has been explained with reference to FIGS. 2a-2 c. Thus,plate 102 which is coupled to the middle portion of the actuatingelement 104 by the rigid shaft 101 is also reciprocally displaced.

As previously mentioned, the actuating element 108 may be constructed inthe same way as the actuating element 104. In this embodiment, theactuating element 108 serves as a force sensor detecting the force whichis required to hold plate 100 in place. For this purpose, preferably oneof the piezoelectric plates 110 acts as an actuating element, whereasthe other one of the piezoelectric plates 110 serves as a deformationsensor element. As can be readily appreciated, both piezoelectric plates110 may serve as an actuating element and an additional deformationsensor element. The deformation sensor element may be a piezoelectricplate or a conventional mechanical deformation sensor, such as a straingage, attached to actuating element 108. The middle portion of actuatingelement 108 will be displaced in response to the shear force applied tothe sample confined between the plates 100 and 102. This displacement isdetected by one of the piezoelectric plates 110 or, alternatively, by anadditionally applied deformation sensing element, and is output as anelectrical signal to a feedback circuitry which is not shown in thefigures. The feedback circuitry, in turn, will supply a voltage to thepiezoelectric plate 110 which serves as an actuating element, so as togenerate a force which opposes the shear force applied by the sample tothe plate 100. Accordingly, the feedback circuitry may be controlledsuch that the actuating element 108, and thus the plate 100, may bemaintained in an undeflected state or any desired position when anoffset voltage is added to the voltage supplied to piezoelectric plate110. Advantageously, the voltage applied to the piezoelectric plate 110,which serves as an actuating element, may also serve as an output of theforce sensor for determining the shear force within the sample. Incomparison with traditional force sensors, this “force rebalance” sensoris exceptionally stiff, i.e. the fixed plate 100 remains fixed for awide range of applied shear forces, and hence it is possible toaccurately determine the shear strain, i.e. the difference in thelateral position between the moving plate 102 and the fixed plate 100.

The output of the force sensor, i.e. the actuating element 108, may thenbe processed in any desired manner, i.e. the voltage obtained from theforce sensor may be amplified, converted into digital signals, stored ina corresponding memory, or processed by a microprocessor so as toreceive required force-displacement curves, which can be related tovarious rheological or mechanical characteristics of the sample, whereinthe dimensions of the plates 100 and 102 are taken into account. In atypical application for polymeric materials, the moving plate 102executes sinusoidal varying motions with frequencies from 0.01-1000rad/s with an amplitude of, at most, 1% of the spacing between theplates 100 and 102. The sinusoidally-varying signal is observed at theforce sensor, wherein the ratio of force waveform amplitude to thedisplacement waveform amplitude is related to the modulus of thematerial at that frequency. The existence of a difference in phasebetween the force and displacement waveforms implies that this modulusmay be represented as a complex quantity. The real part of this complexmodulus corresponds to the “elastic” or “storage” modulus of theviscoelastic material; the imaginary part corresponds to the “viscous”or “loss” modulus.

Although the above embodiment is described using the piezoelectricactuator 104, it is also possible to employ any appropriate means fordisplacing the sample, such as a motorized stage, which then maypreferably include a second force sensor for determining any forceexerted along a line joining the centers of the moving plate 102 and thefixed plate 100 when the plates are positioned above one another. Thistype of force is usually referred to as normal force. A variety of forcesensors are suitable for this purpose, in particular the piezoelectricforce sensor as described above.

With reference to FIGS. 3 and 4, a further embodiment is described whichis capable of generating a rotational displacement with respect to thesurfaces of a small quantity sample.

FIG. 3 shows a perspective schematic view of a rotational embodiment ofthe present invention.

Between a moving plate 300 and a fixed plate 302, a small quantitysample of a material to be characterized may be arranged. Regarding thepreparation of the sample and adjusting the distance between the plates300 and 302, the same considerations as given with respect to thetranslational embodiment shown in FIG. 1 also apply in this case. Movingplate 300 is mounted on a shaft 301 which, in turn, holds an actuatingelement 304. Actuating element 304 is comprised of a metal foil 305 ofrectangular shape. Attached to metal foil 305 are four piezoelectricplates 306, wherein two of the piezoelectric plates 306 are arranged onone surface of the metal foil separated by the shaft 301 and the othertwo of the piezoelectric plates 306 are arranged on the other surface ofthe metal foil. Thus, two of the respective piezoelectric plates 306 arearranged in an opposed relationship with metal foil 305 disposedin-between. The metal foil 305 is clamped in place by a mechanicalassembly provided as clamping blocks 307.

Fixed plate 302 is mounted on a shaft 303 which, in turn, is connectedto a torque sensor 308 which comprises sensor elements 309 that areconnected with one end portion to the shaft 303, and with the other endportion to a support 311.

FIG. 4 shows a schematic top view of the actuating element 304.Similarly, as already explained with reference to FIGS. 2a-2 c, twoopposing piezoelectric plates 306 sandwiching the metal foil 305 areelectrically connected to a voltage supply in such a way thatapplication of a voltage leads, for example, to an expansion of theupper piezoelectric plate and a contraction of the lower piezoelectricplate on the right side of FIG. 4, and the piezoelectric plates 306 onthe left side are accordingly wired so as to exhibit the inversebehavior. Thus, a central axis of the metal foil perpendicular to thedrawing plane of FIG. 4 is subjected to a rotational displacement.

FIG. 5 shows a schematic top view of the torque sensor 308. The shaft303, which may be made of a thermally insulating material, is attached,preferably in a non-permanent manner so as to permit the use ofdisposable plates, to at least two piezoelectric plates. In the presentcase, however, four piezoelectric plates 309 are employed and areattached to the surface of the shaft 303, such that one end of eachpiezoelectric plate 309 is tangential to the surface of the shaft 303.The other end of each piezoelectric plate is tethered to the rigidsupport 311. The crystal structure of plate 309 is aligned such thatapplying a voltage across the plate causes the greatest dimensionalchange along the line tangential to the shaft 303. As is shown, thepiezoelectric plates 309 are preferably aligned along opposite sides ofthe shaft 303 so that the net torque resulting from the application of avoltage across the piezoelectric plates does contain a component whichwould tend to rotate the shaft out of its un-energized orientation.

The torque sensor 308 further contains a sensing element, in this casein the form of the other two of the piezoelectric plates 309. Thesensing element may, however, be any conventional strain gage orpiezoelectric element which generates a signal upon rotation of theshaft 303. A feedback circuit which is not shown in the figure monitorsthis signal and adjusts the voltage applied to the piezoelectric plates309, which act as an actuating element, so as to rebalance the forceapplied to the shaft 303 and to return the shaft 303 to the un-rotatedposition or any desired position when a corresponding offset voltage isadded to the voltage supplied to the piezoelectric plates 309.Advantageously, this voltage may also serve as a measure of the torqueat the shaft 303.

As in the case of the translational embodiment, this design isexceptionally stiff so that the fixed plate 302 remains fixed for a widerange of supplied shear forces. Hence, the torque sensor as describedabove permits the shear strain, i.e. the difference in angular positionbetween the moving plate 300 and the fixed plates 302, to be accuratelydetermined.

FIG. 6 shows an alternate form of the torque sensor. In this alternativeembodiment, the torque sensor 308 comprises two or more piezoelectricassemblies, each consisting of at least one piezoelectric plate 319bounded to a metal foil 315, which are attached to the surface of therigid shaft 303 such that one of the edges of the foil is parallel tothe axis of the shaft 303 and that the other edge of the foil lies alonga line perpendicular to the axis of the shaft. The edge of metal foil315 opposite the edge in contact with shaft 303 is clamped in place bymeans of the rigid support 311. In the embodiment described withreference to FIG. 5, applying a voltage to the piezoelectric platesproduces the expansion or contraction of the assembly along a linetangential to the shaft 303, resulting in a torque. In this alternativeembodiment, applying a voltage to the piezoelectric assembly comprisingthe piezoelectric plate 319 and metal foil 315 produces the buckling ofthe metal foil 315 and thus the rotation o the “shaft end” of theassembly about the “clamped end”, also resulting in a torque beingapplied to the shaft 303. In this embodiment, four piezoelectricassemblies, each comprising a piezoelectric plate and a metal foil areemployed. As it will readily be appreciated, more than one piezoelectricplate 319 per piezoelectric assembly may be used. Moreover, any numberof piezoelectric elements may be employed, wherein advantageously atleast one of the piezoelectric assemblies may be used as a deformationsensing element in combination with a feedback circuit so as to maintainthe shaft 303 on its un-rotated position, thereby providing anexceptionally stiff torque sensor element.

Sample preparation and operation of the rotational embodiment is carriedout in a similar way as described with reference to the translationalembodiment. Although moving plate 300 and fixed plate 302 have beendescribed as plates with flat surfaces, any appropriate geometry of theplate surface may be employed. For example, one of the plates mayconsist of a cone of known apex angle, which is well-known in the art as“cone and plate” geometry.

Furthermore, as already pointed out with reference to the translationalembodiment, the shafts 301 and 303 are preferably made of a rigid,thermally insulating material so that varying the temperature of thesample disposed between the plates 300 and 302 would not be affected bythe shafts 301 and 303 coupled to actuating element 304 and torquesensor 308, respectively. Moreover, the shafts 301 and 303 arepreferably attached to the actuating element and the sensor element in anon-permanent manner so as to permit the use of disposable plates,thereby increasing the speed of sample replacement.

In a further variation, which is not shown in the figures, the rheometercomprises a first plate that is mechanically coupled to an actuatingelement such as actuating element 104, as previously described withreference to FIG. 1. However, any other appropriate actuating elementwhich allows a translational or rotational displacement between thefirst and the second plates may be used. A sensing element, such as adeformation sensing element as described with reference to FIGS. 1-6,may be employed to output a signal to a feedback circuit in response tothe displacement of the first plate. The feedback circuit, in turn,outputs a voltage to the actuating element so as to return the actuatingelement to a predefined position, thereby effecting a force rebalance ofthe force applied to a sample confined between the first and secondplates. The voltage provided to the actuating element may serve as anindication of the shear stress within the sample. As can be readilyappreciated, the second plate can be maintained at a fixed position,either by a second actuating element coupled to the second plate, or afixed support holding the second plate. While the former alternativeprovides for the possibility to perform measurements as described withreference to FIGS. 1-6 and in a way as described in this paragraph, withthe same apparatus, the latter alternative obviates the necessity for asecond actuating element.

Moreover, the displacement can be detected by any suitable encodermeans, such as an optical sensor, etc., to provide the position signalfor the feedback circuit. Furthermore, all of the embodiments asdescribed above may additionally comprise an environmental conditioncontroller 350 (e.g., an environmental chamber), as shown in FIG. 1, soas to vary physical properties, such as pressure, gas composition of anatmosphere surrounding the sample, temperature, electric and magneticfields by providing, for example, electrodes and coils providing anelectrical and magnetic field across the sample, etc. In addition, twoor more of the above embodiments may be combined to form a rheometer“array” or a “parallel rheometer”, wherein advantageously a centralcontrol unit is provided so as to monitor and control the performance ofindividual rheometer elements of the rheometer array. Thus, a largenumber of small quantity samples may be characterized within a shorttime period, wherein the exceptionally stiff force sensor elementsprovide for accurate measurement results, even for the small scaledisplacements required for these samples.

Next, with reference to FIGS. 7-12, a further embodiment of a rheometeris described which allows simultaneous measuring of a plurality ofsamples.

FIG. 7 shows a schematic cross-sectional side view of an exemplaryembodiment of the parallel rheometer of the present invention. In FIG.7, a parallel rheometer 700 comprises a shear plate 701 which maycontain, at predefined locations, raised regions 702 of knowndimensions. Samples 703 are disposed between the shear plate 701 and afixed plate 704 at the predefined regions. In this embodiment, the fixedplate 704 is made of an appropriate substrate carrying correspondingmicromachined sensor elements. However, any appropriate fixed plate,preferably made of a rigid material, such as aluminum or stainlesssteel, may be employed. The shear plate 701 and a fixed plate 704 arearranged parallel to one another with typical plate separations at thepredefined regions of under 1 mm. With each predefined region, i.e. witheach sample, there is associated a sensor element 705 in order to detecta force applied to the sensor element 705 by the shear plate 701 via thesample 703. Although in FIG. 7 a micromachined silicon force sensor isshown as the sensor element 705, any appropriate sensor may be employed,including those sensor elements that are described later with referenceto FIGS. 10-12. The sensor element 705 has been micromachined in asilicon substrate, however, any appropriate material, such as siliconnitride and silicon dioxide, may be used for the fixed plate 704 and thesensor elements 705.

FIG. 8 shows a schematic top view of one rheometer element with shearplate 701 removed. The sample 703 is placed on a rectangular siliconplate of the force sensor element 705. The rectangular plate is attachedto the fixed plate 704, i.e. in this case a silicon substrate, by meansof four tethers 706. The tethers 706 are equipped with piezoelectricmaterial or any other appropriate means for sensing a deformation of thetethers 706.

FIG. 9 is a schematic perspective view of a further embodiment of theparallel rheometer according to the present invention. In FIG. 9, afixed plate 904 comprises predefined regions 902 for receiving a sampleof a material of interest. An array of test fixtures 901 is movablymounted on a means which is not shown in the figure, so that the testfixtures 901 may be lowered onto the predefined region 902 andpositioned at a known distance from the fixed plate 904. The textfixtures 901 acting as actuating elements may have any appropriatelystructured surface so as to define a required shape of theactuator-sample contact. In FIG. 9, a cone-and-plate geometry is shownin which the actuator is a cone of known apex angle. Other geometries,however, such as a parallel plate geometry in which the actuator is aflat disk parallel to the sample surface, may be employed as well. As inthe previously-described embodiment, entrainment of viscous fluids issufficient to keep the samples confined within a column capped by theactuator and the raised regions on the fixed plate 904. The testfixtures 901 are individually coupled to respective motors 910 viarespective encoders 911. Advantageously, the predefined regions 902contain respective force sensor elements.

Again referring to FIGS. 7 and 8, a strain field is generated acrosseach sample by attaching the shear plate 701 to a translation stage 710.This translation stage 710 moves in the plane of the plate-samplecontact at a controlled rate so as to approximate a sinusoidaldisplacement of a required amplitude and frequency. The fixed plate 704remains fixed in position, resulting in a shear field extending througheach sample. Appropriate translation stages providing the requireddisplacement with appropriate amplitude and frequency are well-known tothose skilled in the art.

In a further embodiment, a micromachined electrostatic drive can beassociated with each sample, thereby permitting independent control ofthe strain field for each sample for extremely small displacements. Suchactuators are preferably fabricated from silicon, silicon nitride, orany other suitable materials.

In the embodiment described with reference to FIG. 9, the motors 910 areactuated to provide a rotational displacement which may be provided tothe samples in the form of a sinusoidal displacement of a desiredamplitude and frequency or in the form of a continuous, i.e. anon-reciprocating shear at a defined shear rate. Compared to thetranslational embodiment described with reference to FIGS. 7 and 8, therotational embodiment permits measurements under steady, shearconditions and can also be configured to operate as a controlled stressrheometer, as will be described below.

The translational embodiment as well as the rotational embodimentincludes a shear stress sensor at each sample position. One version ofthis sensor element is described with reference to FIGS. 7 and 8 andconsists of a micromachined silicon rectangle which is tethered to thesurrounding silicon substrate by four micromachined silicon tethers. Thesurface of the rectangle lies in the plane of the surrounding siliconsurface. Applying a shear stress to this rectangle by any of the meansas described above generates a piezoelectric response in the foursilicon tethers which can be detected by conventional electronic meanssuch as a resistance bridge. Any variations in the geometry of thissensor element may be performed so as to permit it to be optimally usedin all of the embodiments described above, i.e. the number of tethers,the shape of the silicon plate for receiving the sample, the location ofthe individual tethers, etc. may be adapted to the type of displacementrequired.

Although the operation of the embodiments of the parallel rheometeraccording to the present invention is described by means of a sinusoidalreciprocating, motion in which, for example, an external processordirects the translation stage attached to the shear plate 701, orindividually directs the micromachined electrostatic actuators, orindividually directs the motors 910 to execute periodic motion in theplane of the plate-sample contact, other types of displacement such astriangular displacement of known amplitude and frequency, in which thestrain rate is constant except at the turning points of the motion, anda motion which approximates a square wave of known amplitude andfrequency may be employed as well. The raw data obtained from the sensorelements consists of the shear stress as a function of time and may bereduced and/or processed so as to yield a single amplitude for the shearstress waveform for each sample, wherein the data processing may beperformed sequentially as well as in a parallel manner, depending on thecomputational ability of corresponding signal processing means. At anyrate, signal processing speed is high compared to the mechanical timeconstant involved in the response of the sample so that a“quasi”-parallel output of measurement results is obtained, even if thesensor signals are sequentially processed.

It is also possible that the samples are subjected to a step shear andthe resulting strain is determined as a function of time. This may beaccomplished by providing the square wave displacement described above.Moreover, the embodiments described above may also be operated in acontrolled stress mode. In this case, an external processor directs theactuator, i.e. the shear plate 701, the micromachined actuators, or themotors 910, to move so as to produce a certain shear stress within eachsample. The displacement required to produce this stress is recordedwith the aid of appropriate means, such as encoders 911. As before, theraw data consists of the displacement as a function of time which may beprocessed in any desired manner.

In other embodiments, the parallel rheometer of this inventionpreferably includes means for controlling and changing environmentalconditions. This may be accomplished by mounting the embodiments in atemperature-controlled environment (not shown in the figures),permitting measurements to be made as a function of temperature, as afunction at one or more temperatures, as a function of one or moretemperature ramp rates, or as a combination of two or more of thepreceding measurement criteria. In this case, temperature sensing can beprovided by a thermocouple, thermistor, or any other appropriatetemperature-sensitive element attached to the shear plate or the fixedplate. Moreover, a thermocouple may be attached to each sensor or astandard micromachined resistor may be associated with each sensor.

Moreover, the embodiments of the parallel rheometer may be mounted in apressure-tight enclosure (not shown in the drawings) permittingmeasurements to be made as a function of pressure, as a function of timeat one or more pressures, as a function of pressure change rate, or as acombination of two or more of the preceding measurement criteria.Preferably, the enclosure is fitted with a purge valve enablingvariation of gas composition and permitting measurements to be made as afunction of composition, of time at a given composition, as a functionof composition change rate, or as a combination of two or more of thesecriteria.

Moreover, either the shear or the sample plate may be fitted with one ormore or an array of electrodes which serve to generate an electric fieldacross each of the samples or across the samples as a whole.Measurements may be made as a function of field amplitude, fieldfrequency, the rates of change of these two quantities, time at a givenvalue of these two quantities, or a combination of two or more of thesecriteria.

In further variation, either plate can include one or more or an arrayof coils which generate a magnetic field across each sample.Alternatively, the entire parallel rheometer may be placed between thepoles of a large magnet or a pair of Helmholtz coils. In this case, thedevices surrounding the sample, i.e. the shear plate 701 and the fixedplate 704 or 904 have to be constructed of a non-magnetic material inorder to avoid eddy currents associated with the motion of the plates.Again, measurements may be made as a function of field amplitude, fieldfrequency, the rates of change of these two quantities, time at a givenvalue of these two quantities, or a combination of two or more of thesecriteria.

Preferably, in the embodiments having electrodes or coils generating amagnetic field, a force sensor element may be employed that is immune toelectromagnetic noise, such as the sensor elements described below.However, the electronic sensor as described above with reference toFIGS. 7 and 8 may, nevertheless, be used as well.

Furthermore, in another embodiment, some or all of the environmentalconditions delineated above may be varied simultaneously.

Next, a variety of force sensor elements are described which may be usedin a rheometer, such as the above-described miniature rheometer and theparallel rheometer of the present invention, or any other appropriaterheometer known in the art.

Some of the sensor elements described below and referred to as“stress-optic” employ a material with a known stress-optic coefficient.This coefficient describes the birefringence, or anisotropic retardationof linearly-polarized light, of a material as a function of the appliedstress per unit path length.

FIG. 10 is a schematic perspective view of the first embodiment of astress-optic sensor according to the present invention. In FIG. 10, anoptical fiber 1001 is attached to a block of stress-optic material 1002.The bock 1002 is made of suitable sensor material, including transparentplastics, such as polymethyl methacrylate, suspensions of liquidcrystals in a polymeric matrix, and certain silica glasses. A fixedplated 1003, preferably made of a rigid material, such as stainlesssteel or aluminum, is attached to the block 1002 in order to receive asample. Opposed to fiber 1001 is a second fiber 1004 which is opticallycoupled to a detector element 1005. Fiber 1001 serves as an input fiberand is designed as a single mode optical fiber allowing the propagationof linearly polarized light. Accordingly, only linearly polarized lightwill enter block 1002, and the polarization direction of the input lightwill be changed in conformity with the shear force applied to the samplewhich is indicated by the arrow in FIG. 10. Preferably, optical fiber1004, acting as an output fiber, is also designed as single mode fiberwith its polarization direction selected such as to be perpendicular tothe polarization direction of the light output by fiber 1001.Preferably, the stress-optic material of block 1002 is prepared so as toexhibit zero birefringence in the absence of stress. Hence,substantially no light will be output by fiber 1004 and input intodetector element 1005 when no shear stress is applied to a sample onplate 1003. Application of stress to the sample and thus to the sensorelement alters the polarization of the light transmitted to block 1002and produces a measurable signal at detector element 1005. The signalincreases with increasing shear stress applied to the sample.

Alternatively, a polarizing element 1006 may be used between fiber 1004and detector element 1005 when input fiber 1001 and output 1004 are ofthe same type, i.e. have the same polarization direction. Polarizingelement 1006 is oriented such that its polarization direction isperpendicular to that of the fibers 1004 and 1001.

In a further alternative, the stress-optic material of block 1002 may bereplaced by a short length of input fiber 1001 or output fiber 1004,which retards the polarization state of the transmitted light beam inresponse to a shear stress applied to the sample.

FIG. 11 schematically shows a further embodiment of a stress-opticsensor element according to the present invention. In FIG. 11, a lightsource 1101, preferable a laser, emits linearly polarized light. Thelight emitted by light source 1101 passes through a half-mirror 1102 andis input into a multimode fiber 1103. A block of stress-optic material1104 is attached to the other end of fiber 1103. The end block 1104opposing the fiber 1103 is provided with a sample plate 1105 which ispreferable made of a rigid material such as stainless steel or aluminum,which is capable of reflecting light. Above sample plate 1005, a“moving” plate 1106, acting as an actuator, is positioned such that asample 1107 is confined between sample plate 1105 and actuator 1106. Inthis embodiment, actuating element 1106 and sample plate 1105 exhibit acone-and-plate geometry, however, any other appropriate geometry, suchas a parallel-plate geometry, may also be used as previously mentioned.Further, in the optical path of light reflected by half-mirror 1102, apolarizing element 1108 is disposed in front of a detector 1109.

In operation, light source 1101, such as a laser, emits linearlypolarized light which passes half-mirror 1102 and enters optical fiber1103 which may be a multimode fiber. The light introduced into opticalfiber 1103 is guided to the stress optic material of block 1104 and isreflected by sample plate 1105. The polarization direction of the lightis altered due to a shear force applied to the sample 1107 by theactuating element 1106 which, in turn, induces a force in thestress-optic material 1104. The reflected light having the alteredpolarization direction due to its interaction with the stress-opticmaterial is partly reflected by half-mirror 1102 and directed topolarizing element 1108 whose polarization direction is perpendicular tothat of light source 1101. Hence, detector 1109 will detect a lightintensity in response to the shear force prevailing in the stress-opticmaterial 1104. Alternatively, optical fiber 1103 may be a single modefiber with its polarization direction adjusted to be parallel with thatof light source 1101. In this case, polarizing element 1108 may beomitted so that detector 1109 will detect a decreasing light intensitywith increasing stress applied to the sample.

Alternatively, referring to FIG. 11B, light from the light source 1101is transmitted through a circular polarizer 1102 b. Circularly polarizedlight is then introduced into the optical fiber and is guided to thestress optic material and reflected back by the sample plate. Thepolarization state of the light is altered due to the stress induced inthe stress-optical material by shear applied to the sample by theactuating element. The reflected light having an altered polarizationstate due to its interaction with the stress-optic material is partlytransferred back through the circular polarizer 1102 b. Hence detector1109 b (positioned beside light source 1101) will detect a lightintensity in response to the shear force prevailing in the stress opticmaterial.

Alternatively, the linearly polarized light may be directed onto thestress-sensor material at an oblique angle. Some fraction of the lightis transmitted through the sensor material and is then reflected fromthe sample plate 1105 to pass back through the stress-optic material. Inthis case, the optical fiber 1103 may be omitted and some fraction ofthe reflected light is transmitted through the material-atmosphereinterface and is incident onto the appropriately-placed polarizingelement 1108 and detector 1109.

In a further embodiment not shown in the Figures, using a single modeoptical fiber as the input, the sensor material and the output fiber arecombined into a single length of a single mode optical fiber. Applying ashear stress to a “sensor” section of this optical fiber rotates thepolarization direction of the light passing through it due to the“stress optic” characteristics of the sensor section, as previouslypointed out. The output section of this single mode optical fibertransmits only that portion of the light which has a polarizationdirection parallel to that transmitted by the input section. Thus, theintensity of light exiting the output section decreases with increasingshear stress applied to the sample. Quantitative measurements may befacilitated by comparing the output of this single mode optical fiber tothat transmitted by a second, unstressed length of a single mode opticalfiber.

Quantitative measurements may be facilitated by comparing the output ofthis single mode fiber optic to that transmitted by a second, unstressedlength of a single mode fiber optic.

Suitable polarizers which may be employed in some of the above-mentionedembodiments include sheets of polarizing films, polarizing mirrors, andthe direction of an initially polarized light beam onto the surface ofthe stress-optic material at the brewster angle. For the detectors usedin the several embodiments mentioned above, films, photomultipliers,avalanche photodiodes, conductive photocells, and CCD cameras may beemployed.

The above-described embodiments of stress-optic sensors according to thepresent invention provide for ease of parallel data acquisition,immunity to electromagnetic noise, and great robustness at lowtemperatures.

With reference to FIG. 12, an improved force sensor element is describedwhich allows the simultaneous measurement of a shear force and a normalforce.

In FIG. 12, a recess portion 1201 is formed in a substrate comprisingsilicon or any other appropriate material, such as silicon nitride orpolyimide, etc., by standard micromachining manufacturing steps, such asphotolithography and etching. Within the recessed portion 1201, arectangular plate 1203 is formed which is tethered by four tethers 1204,1205, 1206, and 1207 to the substrate 1202. Each of the tethers1204-1207 comprises two regions that have piezoresistive properties,which are shown as “N-doped” regions as indicated by “n” in the figure.The tethers 1204 and 1207 are electrically connected by a wiring line1208 which, in turn, is electrically connected to contact pads 1209 and1210, respectively. Similarly, tethers 1205 and 1206 are electricallyconnected by a wiring line 1211 which, in turn, is electricallyconnected to contact pads 1212 and 1213, respectively. FIG. 12 is not toscale and the width of the tethers 1204-1207 is exaggerated incomparison with the side length of the plate 1203. In a preferredembodiment of the force sensor according to the present invention, thewidth of the tethers is about 80 μm and the side length of plate 1203 isin the range of one to several mm. The length of the tethers is about 1mm and the length of a single piezo-resistive area, i.e. of any N-dopedregions in the surface layer of each tether, is about 300 μm.

As the skilled person will readily appreciate, the above dimensions maybe varied in numerous manners so as to satisfy design and applicationrequirements.

The force sensor according to the present invention is preferablymanufactured as an array on a silicon substrate, such as a lightlyN-doped silicon wafer having a thickness of about 400 μm, by means ofconventional semiconductor manufacturing procedures which are well-knownto the person skilled in the art. Hence, a detailed description of thevarious procedural steps in manufacturing the force sensor array will beomitted.

In operation, an input voltage is applied to the contact pads 1209 andan output voltage is obtained at contact pad 1210 which represents themiddle terminal of a piezo-resistive bridge formed by the respectiveN-doped regions of the tethers 1204 and 1207, respectively. Similarly,an input voltage, possibly of the same amount as that applied to contactpads 1203, is applied to contact pads 1212 and an output voltage can bedetected at contact pad 1213 which represents the middle terminal of apiezo-resistive bridge formed by the respective N-doped regions on thetethers 1205 and 1206, respectively. When a shear force is applied tothe plate 1203, i.e. a force which, in the configuration of FIG. 12, issubstantially oriented in the plane of the plate 1203 and normal to theaxes of the tethers 1204-1207, so as to cause a slight displacement ofthe plate 1203, thereby generating a deformation of the tethers 1204 to1207. The N-doped regions of tether 1207 are mirror-symmetricallyarranged with respect to a vertical middle axis of plate 1203 so thatthe change in resistance of the tethers 1204 and 1207 will substantiallycancel out each other. Accordingly, the voltage detected at contact pad1210 substantially remains unchanged.

Contrary to this, on the tethers 1205 and 1206, N-doped regions arevertically arranged side by side so that upon application of a shearforce, a maximum change of resistance in the doped regions of tether1205 and 1206 will be obtained. Moreover, the doped regions on tether1205 are inversely arranged to that of tether 1206 so that a maximumshift of the voltage detected at contact pad 1213 will occur.Accordingly, a shear force applied to the plate 1203 will provide asubstantially unchanged output voltage on contact pad 1210, irrespectiveof the magnitude of displacement of plate 1203, and a maximally-shiftedoutput voltage on contact pad 1213 depending on the magnitude ofdisplacement. Hence, tethers 1205 and 1206 act as a shear force sensorelement.

Similarly, when a force is applied to the plate 1203 which is directedperpendicular to the drawing plane of FIG. 12, the tethers 1204-1207 aredeformed in such a way that the output voltage on contact pad 1210 ismaximally shifted depending on the magnitude of displacement of plate1203 in the direction perpendicular to the drawing plane, whereas theoutput voltage detected at contact pad 1213 remains substantiallyunchanged, irrespective of the magnitude of the displacement.Accordingly, the tethers 1204 and 1207 act as a normal force sensorelement.

Although the force sensor which is able to simultaneously detect a shearforce and a normal force has been described with reference to theembodiment as shown in FIG. 12, a variety of modifications may beperformed still providing the same advantages as the embodimentdescribed above.

For example, although the shear and normal force sensor element has beendescribed to comprise four tethers, each including fourappropriately-doped resistive regions, it is not necessary to providefour tethers per sensor plate. In certain circumstances, it may bepreferable to have merely one tether for the normal force sensorelement, or the shear force sensor element, or both. In this case, acorresponding displacement of the plate 1203 causes a deformation of thefour-doped regions on each tether. When a shear force is applied, thedeformation of the doped regions, which are arranged as shown, forexample, on tether 1205 will lead to a change of resistance, which canbe detected when a constant current is supplied to the tether.Similarly, a normal force applied to the tether will result in a changeof resistance when the tether has an arrangement as shown with referenceto tether 1204.

Moreover, in order to provide a stiffer behavior of the sensor elementin responding to the shear force, it may be advantageous to arrange thetethers and doped regions thereon in such a way that the shear force isapplied in the longitudinal direction of the tethers. For this case,tether 1207 of FIG. 12 will be doped as is shown in the figure andtether 1206 will be doped as tether 1204 of FIG. 12, wherein contactpads 1209 and 1212 are electrically connected so as to serve as a middleterminal of the normal force resistant bridge. The shear force bridgeformed of tethers 1204 and 1205 include, respectively, at least onedoped region, wherein contact pads 1209 and 1212 are connected to serveas a shear force output. An input voltage common to the shear forcesensor bridge and the normal force sensor bridge is applied at thecontact pads 1210 and 1213.

Furthermore, the geometric arrangement of the tethers and the plate maybe adapted so as to appropriately detect a rotational displacementapplied to the plate 1203. This may include an arrangement in which thesample plate 1203 is tethered to the supporting substrate 1202 bydiagonally-arranged tethers. Moreover, the plate 1203 may be formed in acircular shape and the tethers, provided in any appropriate number, mayradially extend to the surrounding substrate.

The preferred layout of N-doped piezo-resistor regions shown in FIG. 12is preferably aligned along the direction of maximum tension andpreferably along the direction of maximum change in piezo-resistance, toget maximum sensitivity. For the embodiment shown in FIG. 12, the 100direction is the direction of both maximum tension and maximumpiezo-resistance change. Although the use of N-doped regions is shown,all N-doped regions may be replaced with P-doped regions. Also, thedevice alternatively may have different orientations of thepiezo-resistor. For example, a P-doped piezo-resistor would preferablybe aligned along the 110 direction, at 45 degrees to the direction oflongitudinal forces for maximum sensitivity. In alternative embodimentsfor the FIG. 12 embodiment of this invention, the N-doped regions may bereplaced with any suitable piezo-resistive element, for example,piezo-resistive metal wires or doped polycrystalline silicon.

In order to achieve accurate measurement results, preferably each sensorelement is calibrated by applying a defined normal force and shear forceand detecting the corresponding output voltages so as to determine thedegree of mixture of these two force components for each sensor device.

In an alternative embodiment, not shown in the figures, a device thathas both piezo-resistive readout as well as capacitive electrodes tomeasure non-uniform forces acting on the plate may be used. In thepreferred embodiment, four capacitive bond pad areas are placed at thefour corners of the floating plate. Using both the piezo-resistor andthe capacitive readouts, all the forces including the shear, the normalforces, and the rotational forces could be deconvolved. A skilledpractitioner of the art can change the capacitor placement as well aschange the capacitance area to maximize the device sensitivity.

Since the sensor element according to the present invention ispreferably manufactured as an array of sensor elements on an appropriatesubstrate, this sensor element is advantageously used in combinationwith a parallel rheometer as previously described. Moreover, since thesesensor elements may be produced in mass production at low cost, thesensor element may be designed as a disposable device so as tosignificantly facilitate sample preparation for a parallel rheometer.The inventive sensor element, however, may as well be used incombination with a single sample rheometer.

What is claimed is:
 1. A parallel plate rheometer for simultaneouslyanalyzing material characteristics of two or more samples, comprising:first and second plates, respectively having regions, for receiving andconfining said two or more samples, the first and second plates beingmoveable relative to each other; an actuator adapted to move the firstand second plates relative to each other for producing a shear strainwithin each sample; and at least one sensor associated with each regionfor simultaneously detecting shear stress within each sample.
 2. Theparallel plate rheometer of claim 1, further comprising an environmentalcondition controller applying varying environmental conditions to thetwo or more samples.
 3. The parallel plate rheometer of claim 2, whereinthe environmental condition controller is adapted to vary at least oneof temperature, pressure at a fixed gas composition, composition of agas atmosphere, electric field, magnetic field; and time of applicationof one of the preceding quantities when adjusted to a predeterminedvalue.
 4. The parallel plate rheometer of claim 2, wherein theenvironmental condition controller is adapted to individually vary theenvironmental conditions of at least one of the samples.
 5. The parallelplate rheometer of claim 1, wherein the first and second plates comprisea shear plate and a fixed plate which are arranged in a parallel mannerto each other and which are separated from each other by an adjustabledistance.
 6. The parallel plate rheometer of claim 5, wherein at leastthe shear plate comprises a raised region of predetermined dimension soas to confine a sample between the shear plate and the fixed plate. 7.The parallel plate rheometer of claim 5, comprising a translation stagecoupled to the shear plate for moving the shear plate linearly and in aparallel manner with respect to the fixed plate in accordance with arequired type of motion.
 8. The parallel plate rheometer of claim 1,wherein the shear stress sensor comprises a micromachined silicon platewhich is tethered to a surrounding substrate by at least two tethers,each having a piezo-resistive element responsive to a deformation of thetether.
 9. The parallel plate rheometer of claim 8, wherein thepiezo-resistive element assists in generating a signal proportional tothe deformation of the tethers.
 10. The parallel plate rheometer ofclaim 1 wherein the shear stress sensor comprises a stress-sensingmaterial of a defined stress-optic coefficient indicating one ofbirefringence and retardation of linearly polarized light passing thestress-sensing material, as a function of applied stress/unit pathlength.
 11. The parallel plate rheometer of claim 10, wherein polarizedlight is input into said stress-sensing material, and light havingpassed the stress-sensing material is guided through a polarizer meanshaving a polarization direction different from and preferablyperpendicular to that of the input light.
 12. The parallel platerheometer of claim 10, wherein the input light is polarized by a singlemode input fiber.
 13. The parallel plate rheometer of claim 12, whereinsaid stress-sensing material is a short portion of the single mode inputfiber.
 14. The parallel plate rheometer of claim 10, wherein a singlemode fiber comprises a light input portion, a middle portion as thestress-sensing material, and a light output portion.
 15. The parallelplate rheometer of claim 10, wherein a surface of the stress-sensingmaterial, which may be brought into contact with a sample is providedwith a reflective layer, and the polarized light is directed to thestress-sensing material by an oblique angle with respect to thereflective layer, whereby the polarizer means is positioned to receivelight reflected by the reflective layer.
 16. The parallel platerheometer of claim 10, further comprising a detector arranged so as todetect a signal output by the stress-sensing material.
 17. The parallelplate rheometer of clam 1, wherein the actuator comprises an actuatorelement for each sample which is symmetrical with respect to an axisnormal to a surface of the sample, wherein the actuator element isrotatable around said normal axis by a driving means.
 18. The parallelplate rheometer of claim 17, wherein the driving means comprises a motorand an encoder for generating a rotational displacement of each of theactuator elements.
 19. The parallel plate rheometer of claim 1, whereinthe at least one sensor comprises a normal force sensor detecting aforce component which is perpendicular to a shear force.
 20. Theparallel plate rheometer of claim 19, wherein the normal force sensor isprovided in each region.
 21. The parallel plate rheometer of claim 19,wherein the normal force sensor is integrated in a shear force sensorprovided at each region.